This invention relates to precision casting, and, more particularly, to an improved method for making a unitary precision investment casting mold.
One form of an investment casting mold which is generally used to make relatively small metal castings is the type which includes as part of the mold a charge-holding and melting chamber in an upper portion of the mold. Such a chamber is in communication with lower portions of the mold which include a cavity in which the metal article is to be produced. As a part of the well-known precision investment casting process, a metallic charge is placed in solid form within the charge-holding and melting chamber. Then the solid charge is melted and the molten metal drops into the lower portions of the mold. This eliminates the need for including separate molten metal pouring apparatus which can be costly and require substantially larger apparatus when the precision investment casting process is conducted in a vacuum.
One form of this general type of method and mold is described in U.S. Pat. No. 3,435,878 -- Howard et al., patented Apr. 1, 1969. As is shown in that reference, it is sometimes desirable to withhold deposition of the molten metal in the charge-holding chamber, for example to provide for complete melting and homogenization of the charge. This can be accomplished by the introduction between the charge and the lower portion of the mold of a metal plug or slug which melts at a temperature higher than that of the principal charge.
In order to provide such fusible plug between the charge-holding chamber or crucible and the lower portion of the mold, some molds have been made with separate crucibles, for example as shown in FIGS. 4, 5, 6 and 7 of the above-described Howard et al. patent. In such cases of non-unitary molds, the plug is either placed between the two portions or is embedded in the ceramic of the mold. In the unitary molds shown in FIGS. 8 and 9 of that patent, the plug is placed between two wax portions from which the mold is made.